action_sheet: Remove backward-compat code for old special edit-limit value

Fixes-partly: zulip#1837

Author: chrisbobbe

lib/widgets/action_sheet.dart

@@ -1146,10 +1146,7 @@ bool _getShouldShowEditButton(BuildContext pageContext, Message message) {
 
   final now = ZulipBinding.instance.utcNow().millisecondsSinceEpoch ~/ 1000;
   final editLimit = store.realmMessageContentEditLimitSeconds;
-  final outsideEditLimit =
-    editLimit != null
-    && editLimit != 0 // TODO(server-6) remove (pre-FL 138, 0 represents no limit)
-    && now - message.timestamp > editLimit;
+  final outsideEditLimit = editLimit != null && now - message.timestamp > editLimit;
 
   return message.senderId == store.selfUserId
     && isComposeBoxOffered

test/widgets/action_sheet_test.dart

@@ -2196,8 +2196,6 @@ void main() {
         }
 
         testVisibility(true);
-        // TODO(server-6) limit 0 not expected on 6.0+
-        testVisibility(true, limit: 0);
         testVisibility(true, limit: 600);
         testVisibility(true, narrow: ChannelNarrow(1));